The present invention relates to a sputtering apparatus and more particularly, to a sputtering apparatus adapted to form thin films on angular substrates of a large area uniformly at high speeds.
A sputtering apparatus has been often used to manufacture semiconductor devices, optical discs, liquid crystal, or electronic components, etc., because of its capability to form a thin film of a target material stably on a substrate. As performance capabilities required for this kind of sputtering apparatus, speed and cost are important, that is, how quickly and stably the apparatus forms a uniform film all over the substrate at low costs.
Meanwhile, a stationary formation method has been increasingly employed in recent sputtering apparatuses, besides a substrate-moving method whereby a film is formed on a target while the substrate is moved to the target. The substrate is laid still and opposed to the target in the stationary method. The stationary method advantageously improves a film-forming speed, decreases dusts, and reduces an equipment cost.
During sputtering, atoms sputtered from on the target are radiated with a certain angle distribution. Therefore, it is necessary in the stationary method to optimize a shape of the target, structures, and sizes of magnets, etc. so as to fulfill the aforementioned performance capabilities in accordance with a shape and a size of the substrate to which a thin film is formed.
Since circular substrates represented by silicon wafers have been used for manufacturing semiconductor devices and optical discs, the sputtering apparatus of the stationary model has employed a disc-like target. A ring-shaped eroded part (referred to as an "erosion" below) is formed on the circular target to secure a uniform thickness of the thin film formed on the substrate, which ensures a thickness uniformity all over the surface of the circular substrate.
The above-referred erosion is generated when the plasma is locally distributed in consequence of the drift motion of electrons on the target surface. Therefore, a fixed magnet having a ring-shaped distribution of magnetic field is arranged below the target to realize the erosion in the form of a ring. Since the electrons are trapped while drifting within the ring in this case, the plasma is generated with a high density, so that many electrons are sputtered and a film is formed at high speeds.
In the meantime, when a film is to be formed on an angular substrate of a display device or the like, a fixed magnet to generate a linear erosion on an angular target is disposed below the target thereby to assure a film thickness distribution on the angular substrate.
However, the obtained linear erosion is not sufficient to confine the plasma through the drift of electrons, eventually making it hard to form the film at high speeds. As such, the magnet is generally constructed in such configuration that two linear erosion parts parallel to each other are connected at both ends thereof by semi-circular erosion parts. The electrons can thus be trapped in the same fashion as when the ring-shaped target is used, and a high-density plasma is generated thereby to form the film quickly.
In the case of forming a film on an angular substrate of a larger area in comparison with that of the above target, while the angular substrate with a large area is held by a substrate holder which is movable facing to the target, the substrate holder is moved thereby to form the film continuously. Thus, the film can be formed on the angular substrate of a larger area in comparison with that of the target. When the substrate holder is moved in parallel to the surface of the target, it is called as a tray system. On the other hand, when the substrate holder is rotated with a radius in a vertical direction of the target surface, this is called as a carousel system.
When the film formation is carried out by moving the substrate as above, the film is formed to the substrate holder as well, resulting in disadvantages of an increase of dusts as the film of the substrate holder is separated or an increase of mechanism elements in a vacuum which leads to an increase of the equipment cost, etc.
Under such being the circumstances, similar to the circular substrate, the angular substrate of a large area has been started to be treated in the stationary state.
A conventional method of forming a thin film on the angular substrate of a large area in the stationary state will be described with reference to the drawings.
FIG. 7 is a perspective sectional view showing the basic structure of a conventional apparatus forming a film to a large-area, angular substrate in the stationary method with the use of sputtering.
In FIG. 7, a reference numeral 12 is a magnet which is movable on a rear surface of a target 11 in parallel to the surface of the target 11. Other reference numerals are respectively: 13 a large-area glass substrate; 14 a DC power source for impressing power to the target 11; 15 a line of magnetic force generated on the target 11 by the magnet 12; and 16 atoms of a target material sputtered from on the target 11.
How the film-forming apparatus constituted as above operates to sputter in the stationary method will now be depicted.
Sputtering is a way of forming a thin film having a composition of a target 11 on a substrate 13. Concretely, while an inert gas such as argon gas or the like is introduced in a vacuum chamber (not shown), the electric power is supplied from the power source 14 to an electrode part (cathode) including the target 11. The introduced gas is turned into a plasma state, sputtering the material of the target 11 by means of gas ions, thereby forming a thin film on the substrate 13.
In the apparatus of FIG. 7, in addition to the above-discussed principle of sputtering, the magnet (permanent magnet) 12 is disposed at a rear surface of the target 11 to generate lines of magnetic force as indicated by the dotted lines 15 on a front surface of the target 11, and the electrons (not shown) as a cause to generate plasma are thus confined in an area surrounded by the lines 15. As a result of this, the plasma is locally generated centering a part where components of each line 15 of magnetic force parallel to the target 11 are zero, and the target 11 is consequently sputtered by a lot of gas ions. Thus, since an amount of sputtered atoms 16 is increased, a film-forming speed is improved. As a result, an erosion as a local eroded part is formed on the sputtered target 11.
FIG. 8 is a simulation result of a thickness distribution of the film formed on the substrate 13 when the magnet 12 is set still at the rear surface of the target 11 on a center point of the substrate 13.
A position within a plane of the substrate 13 is indicated by its distance from the center point of the substrate 13 in X- and Y-axes directions in FIG. 8. A relative thickness of the film at the position is shown on a Z-axis.
As is understood from FIG. 8, the thickness distribution in the Y-axis direction can be uniformed by optimizing a distribution of magnetic field generated by the magnet 12. However, the thickness in the X-axis direction is rapidly decreased as the position of the film is far from a central part (X=0) of the magnet 12.
As a way to obtain a uniform thickness distribution all over the surface of the substrate 13, the magnet 12 may be enlarged in size thereby to increase a uniform thickness area in the X-axis direction. However, a limit of a holding force of the magnet 12 weakens the line 15 of magnetic force generated on the surface of the target 11 if a distance between the N and S poles of the magnet 12 is increased. An effect of the magnet to confine the plasma tends to be null.
In the prior art, therefore, the magnet 12 is let to slide in the X-axis direction facing to the substrate 13 at the formation of the film, with the result that the film is obtained with a thickness distribution of an integration in time in the X-axis direction of the thickness distribution when the magnet 12 is still. The thickness distribution of the film all over the surface of the substrate 13 is made uniform in this manner.
In the above-described conventional method, although the thickness distribution in the Y-axis direction on the substrate 13 is stable because it is determined by characteristics of the magnet 12, that in the X-axis direction is greatly changed depending on the control of a speed distribution when the magnet 12 is slid, thereby making it difficult to form the film stably.
If it is so arranged as to form the film on the substrate 13 only when the magnet 12 is slid at a constant speed, the film is actually formed relatively stably. However, this arrangement necessitates a space of a width of the magnet 12 in the X-axis direction to allow the magnet 12 to move, at both ends on the substrate 13. In other words, the apparatus including the target 11 and the vacuum chamber, etc. becomes bulky in size.
As compared with the film-forming speed when the magnet 12 is kept still, the speed is decreased when the magnet 12 is slid, and therefore more electric power becomes required to improve the film-forming speed, that is, the power source 14 should be designed to produce considerably large power.
In the case where the target 11 is metallic, plasma can be obtained by discharging with the use of the DC power source 14. However, if the target is made of an insulating material such as an oxide or the like, the discharge is not performed by the DC power source 14 and therefore, an RF power source or the like high frequency power source is generally employed in place of the DC power source 14. In such case, a matcher for matching an impedance is set between the power source 14 and the target 11 to realize discharging of electricity. However, since a surface state of the target 11 is changed or a change is caused in shape of the erosion subsequent to the slide of the magnet 12 during discharging, the matching state is not stable and the film-forming speed is varied.
Further, in order to form the film at a uniform speed, the target 11 should be an integral body of an area about three times the area of the substrate 13. Therefore, the vacuum chamber becomes large in size, the material cost is raised and the maintenance work to exchange the exhausted target 11 needs an extensive process.